The thrust of this proposal is to continue and expand an analysis of eukaryotic transcriptional activation at the molecular level. The model system employed is the CYC1 gene (iso-1-cytochrome c) of the yeast Saccharomyces cerevisiae. The transcriptional activators that regulate this gene, HAP1, HAP2, HAP3, and HAP4, have many features in common with activators from higher cells. These features include mechanisms of activation the transcriptional machinery, and regulation by small effectors and by post-translational modification. In higher cells, transcriptional activators play key roles in growth control and cancer (several oncogenes are activators of transcription), and are involved in key decisions in embryonic development. The specific aims of the proposal include studies of the yeast activators using a combined approach of biochemistry, molecular biology, molecular genetics, and classical yeast genetics. Further, certain experiments will provide and entree' into studies of related molecule in animal cells. It is hoped that these studies will contribute to a coherent and unified understanding of how eukaryotic activators work and how they are regulated.